Microcellular plastic foam refers to a polymer that has been specially foamed to thereby create micro-pores or cells (also sometime referred to as bubbles). The common definition includes foams having an average cell size on the order of 10 microns in diameter, and typically ranging from about 0.1 to about 100 microns in diameter. In comparison, conventional plastic foams typically have an average cell diameter ranging from about 100 to 500 microns. Because the cells of microcellular plastic foams are so small, to the casual observer these specialty foams generally retain the appearance of a solid plastic.
Microcellular plastic foams can be used in many applications such as, for example, insulation, packaging, structures, and filters (D. Klempner and K. C. Fritsch, eds., Handbook of Polymeric Foams and Foam Technology, Hanser Publishers, Munich (1991)). Microcellular plastic foams have many unique characteristics. Specifically, they offer superior mechanical properties at reduced material weights and costs.
The process of making microcellular plastic foams has been developed based on a thermodynamic instability causing cell nucleation (J. E. Martini, SM Thesis, Department of Mech. Eng., MIT, Cambridge, Mass. (1981)). First, a polymer is saturated with a volatile foaming agent at a high pressure. Then, by means of a rapid pressure drop, the solubility of foaming agent impregnated within the polymer is decreased, and the polymer becomes supersaturated. The system is heated to soften the polymer matrix and a large number of cells are nucleated. The foaming agent diffuses outwards and into a large number of small cells. Stated somewhat differently, microcellular plastic foam may be produced by saturating a polymer with a gas or supercritical fluid and using a thermodynamic instability, typically a rapid pressure drop, to generate billions of cells per cubic centimeter (i.e., bubble density of greater than 108 cells per cubic centimeter) within the polymer matrix.
Conventional solid-state microcellular processing is known to involve a two-stage batch process. In the first stage (absorption), a solid polymer is saturated with high pressure inert gas (e.g., CO2) in a pressure vessel until a desired gas concentration level is achieved throughout the polymer matrix. Once the gas-polymer mixture is removed from the pressure vessel into ambient environment (desorption), a supersaturated specimen is produced that is thermodynamically unstable due to the excessive concentration of gas in the polymer. In the second stage (foaming), the gas-polymer mixture is heated in a hot water bath or some other heating medium (e.g., hot air, steam, infrared radiation, etc.) at a temperature close to the glass transition temperature (Tg) of the gas-polymer mixture in order to induce microcellular bubble nucleation and growth.
The success of the batch process in producing discrete units of thermoplastic material has not, however, been duplicated in large scale production involving continuous rolls, sheets or films of thermoplastic material. To scale-up the batch process for industrial production, several patented methods have been issued for thermoplastic processing. Exemplary in this regard are the following:
U.S. Pat. No. 5,158,986 to Cha et al. (issued Oct. 27, 1992) discloses the formation of microcellular plastic foams by using a supercritical fluid as a blowing agent. In a batch process, a plastic article is submerged at pressure in a supercritical fluid for a period of time, and then quickly returned to ambient conditions so as to create a solubility change and nucleation. In a continuous process, a polymeric sheet is extruded and run through a system of time-controlled rollers within a container of supercritical fluid at pressure, and then exposed quickly to ambient conditions. Dynamic seals are stationed between the chambers to allow passage of the thermoplastic sheet while preserving the environmental conditions of each chamber.
The breakthrough in large scale solid-state microcellular thermoplastic production is disclosed in U.S. Pat. No. 5,684,055 to Kumar et al. (issued Nov. 4, 1997), which patent discloses a method for the semi-continuous production of microcellular foamed articles. As disclosed, a roll of polymer sheet is provided with a gas channeling means (e.g., gauze, paper towel) interleaved between the layers of polymer. The interleaved roll is exposed to a non-reacting gas at elevated pressure for a period of time sufficient to achieve a desired concentration of gas within the polymer. The saturated polymer sheet is then separated from the gas channeling means and bubble nucleation and growth is initiated by heating the polymer sheet (FIG. 1). A limitation of the semi-continuous method, as acknowledged by Kumar, is that only a finite length of solid thermoplastic material may be processed at one time (to ensure that it is foamed promptly before too much gas escapes the material during its time under ambient conditions, a factor that could lead to undesirable variations in foam density).
U.S. Patent Application Publication No. US2005/0203198 to Branch et al. (published Sep. 5, 2005) discloses another semi-continuous solid-state process that utilizes gas impregnation (similar to that of Kumar et al.) under specialized conditions to enhance foaming and thermoforming of the thermoplastic material.
While the semi-continuous methods as taught by Kumar and Branch address one factor associated with uneven gas concentrations of a thermoplastic roll (namely, within-roll variation in absorption rates of exposed vs. non-exposed surfaces to high pressure gas), there are still other factors that can cause unwanted variations in gas concentration during the absorption or desorption phase or both (which may result in unevenly foamed thermoplastic products). For example, within-roll variation in gas concentration during absorption is believed to be, in part, a function of the stress and volume dilation of the gas impregnated roll (and is applicable to all polymer types).
In addition, high pressure gas saturation of an interleaved thermoplastic roll may also cause: (1) the thermoplastic roll to become heavier and softer (hence weaker), thereby resulting in sagging and greater stress at the top portion of the roll (FIG. 2) and even further within-roll variation in gas absorption rates and gas concentration levels between the top (A) and bottom (B) portions of the roll; (2) volume dilation of the roll whereby the roll expands in volume and begins to compress the interleaved medium to the extent that it loses porosity and, consequently, its gas-permeation function.
Moreover, a saturated interleaved thermoplastic roll with fast gaseous diffusion, whether due to the class of polymer, like polylactic acid (PLA) and polystyrene (PS), or due to the thin dimension (<0.010 inch) of a polymer with moderate diffusivity, tends to desorb gas quickly once it is moved into ambient environment and must be heated substantially immediately to obtain even foaming. If the time between absorption and foaming exceeds the narrow window of processability, within-roll variation of gas concentration across the length of the roll, relative to which end is heated first, may lead to uneven bubble growth and size resulting in a non-uniform microcellular foamed structure and non-uniform density.
Saturated interleaved thermoplastic rolls with moderate gas diffusivity, like polyethylene terephthalate (PET) and polycarbonate (PC), are allowed a longer time between absorption and foaming because they desorb gas at slower rates under ambient conditions. However, between-roll variation in gas concentration may nevertheless occur if a batch of thermoplastic rolls of moderate gaseous diffusion, which have been saturated with gas and removed from the pressure vessel at the same time, sit too long under ambient conditions on queue to be heated. Each thermoplastic roll that is subsequently foamed has been exposed to ambient conditions longer and thus may experience incrementally higher reductions in gas concentration. Significant variation in gas concentration between successively heated rolls my result in uneven foam quality among the batch of foamed thermoplastic rolls. The problem of uneven gas concentration in the continuous production of solid-state microcellular thermoplastics is due, in part, to limitations of existing methods and apparatuses (meaning that such methods and apparatuses are not designed to control for any untoward physical events in the thermoplastic material during absorption nor to respond to downstream processing flow by regulating the desorption time). For instance, current pressure vessels used to saturate thermoplastics with high pressure gas are typically designed as a single-chamber cavity with a single-door opening to allow for the insertion and removal of the treated sample. In one embodiment of a single-door, single-chamber pressure vessel, a plurality of interleaved thermoplastic rolls are housed inside the pressure chamber where one or more inlet valves inject high pressure gas (e.g., CO2) into the chamber saturating the thermoplastic rolls until they obtain the desired gas concentration level. Outlet valves then evacuate the gas from the chamber and the door is swung open to remove the saturated rolls from the pressure vessel. This single-door, single-chamber pressure vessel, which constrains the absorption process to a uniform time frame where interleaved thermoplastic rolls are placed inside the pressure vessel at T1, saturated with gas at T2, and evacuated at T3, is not designed for time-sensitive, downstream processing flow.
In another embodiment of the single-door, single-chamber pressure vessel, a plurality of interleaved thermoplastic rolls that have been saturated inside the pressure chamber are taken out of the pressure vessel at different times. Before any of the interleaved thermoplastic rolls can be removed, the pressure vessel must first be depressurized and evacuated of gas. This change inside the pressure vessel environment means that the processing condition of the remainder rolls has been significantly interrupted by depressurization, gas desorption, and subsequent repressurization. In this instance, the single-door, single-chamber pressure vessel is unequal to the demands of production flow while maintaining a constant pressurized environment.
Accordingly, there is a need in the art for novel methods and apparatuses for continuous production of foamed thermoplastic material with consistent quality in microcellular structure and foam density. The present invention fulfills these needs and provides for further related advantages.